Purification of carbon dioxide

ABSTRACT

A first contaminant selected from oxygen and carbon monoxide is removed from impure liquid carbon dioxide using a mass transfer separation column system which is reboiled by indirect heat exchange against crude carbon dioxide fluid, the impure liquid carbon dioxide having a greater concentration of carbon dioxide than the crude carbon dioxide fluid. The invention has particular application in the recovery of carbon dioxide from flue gas generated in an oxyfuel combustion process or waste gas from a hydrogen PSA process. Advantages include reducing the level of the first contaminant to not more than 1000 ppm.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of application of Ser. No. 11/656,914 filed on Jan.23, 2007 which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a process and apparatus forpurification of impure liquid carbon dioxide (“CO₂”) comprising a firstcontaminant selected from the group consisting of oxygen (“O₂”) andcarbon monoxide (“CO”). The process and apparatus have particularapplication in the recovery of carbon dioxide from waste carbon dioxidegas, for example flue gas from an oxyfuel combustion process or wastegas from a hydrogen (“H₂”) pressure swing absorption (“PSA”) process.

There is an urgent need to develop new processes for production ofelectrical energy from fossil fuels, carbonaceous fuels or hydrocarbonfuels with capture of carbon dioxide. The new processes should ideallybe more efficient and cost effective than existing processes. Oxyfuelcombustion processes are being considered in this context.

In oxyfuel combustion, a fuel is combusted in pure oxygen with optionalrecycle of cooled flue gas or steam or water to moderate the flametemperature. The elimination of the bulk of the nitrogen from thecombustion results in a net flue gas which has a high carbon dioxideconcentration following cooling and water condensation.

An oxyfuel combustion process is ideally suited for use in aconventional pulverized coal fired boiler for generation of steam usedfor electric power production. The use of oxyfuel combustion in apulverized coal fired boiler results in a net flue gas production which,after cooling and condensation of contained water vapor, typicallycomprises from about 65 mol % to about 95 mol % carbon dioxide and up toabout 5 mol % oxygen with the majority of the remainder being nitrogenand argon. The oxygen, nitrogen and argon are referred to as“contaminant gases”.

The bulk of the oxygen in the flue gas originates from the excess oxygenrequired for complete coal combustion. The remaining oxygen originatesfrom air leaking into the boiler and convection section. The nitrogenand argon in the flue gas originates from the oxygen feed for coalcombustion, which would typically have a purity of 90 mol % to 99.6 mol%, and usually 95 mol % to 97 mol %, oxygen, and from air leaking intothe boiler and convection section.

Also present in the flue gas are impurities such as acid gases and otherimpurities derived from the coal and the combustion process. Theimpurities include sulfur dioxide, sulfur trioxide, hydrogen fluoride,hydrogen chloride, nitric oxide, nitrogen dioxide, mercury, etc. Thetotal amount of these impurities in the flue gas (after washing anddrying) depends on the composition of the fuel and the combustionconditions.

The flue gas must be purified before carbon dioxide from the flue gascan be stored in, for example, geological formations. In thisconnection, water soluble components such as sulfur trioxide, hydrogenchloride and hydrogen fluoride, are usually removed from the flue gas bydirect contact with water which not only washes out these components butalso cools the flue gas and condenses water vapor. Sulfur dioxide andthe oxides of nitrogen may be removed during compression of the carbondioxide to pipeline pressure as disclosed in U.S. patent applicationSer. No. 11/287,640 filed on 28 Nov. 2005, the disclosure of which isincorporated herein by reference. This process also removes any mercurythat may be present in the carbon dioxide.

The pipeline pressure of carbon dioxide will usually be from about 100bar to about 250 bar which is well above the critical pressure of carbondioxide. The bulk of the contaminant gases is preferably removed toreduce the power required to compress the carbon dioxide and to ensurethat two phase flow conditions do not arise in the pipeline or in thegeological formation in which the carbon dioxide is to be stored.

The presence of oxygen may present problems when the carbon dioxide isintended for use in enhanced oil or gas recovery operations due to thepossibility of oxidation causing corrosion problems in downholeequipment. The typical specifications for carbon dioxide purity would bea maximum contaminants level of 3 mol % and, in the case of the use ofcarbon dioxide for enhanced oil recovery, the maximum oxygen contentwould be typically 100 ppm or lower, even as low as 1 ppm.

The current technology for the next stage of carbon dioxide purificationuses a technique in which the contaminant gases are removed from thecompressed dried pre-purified crude carbon dioxide stream at about 30bar pressure by cooling the crude carbon dioxide to a temperature veryclose to the freezing point of carbon dioxide, where the carbon dioxidepartial pressure is from about 7 bar to about 8 bar. The residual gascontaining about 25 mol % carbon dioxide is separated and vented afterheating and work expansion to produce power. This single process resultsin a carbon dioxide recovery of about 90%. The process of oxyfuelcombustion would be considerably improved if very high carbon dioxiderecoveries, e.g. above 97%, could be achieved economically.

The current technology for delivery of carbon dioxide from the oxyfuelcombustion of fossil fuel to a geological storage site is based oncompression to a pipeline pressure of typically about 100 bar to about250 bar. An alternative technology for smaller sources of carbon dioxideemission, or where a pipeline might be too expensive, is to liquefy thecarbon dioxide and transport the carbon dioxide at a pressure below itscritical pressure as a liquid in, for example, a large seaborne tanker.The oxyfuel combustion process would be significantly improved if thecarbon dioxide purification process could produce economically a liquidcarbon dioxide product rather than a stream of supercritical carbondioxide at near ambient temperature for pipeline delivery.

An important objective for carbon capture in an oxyfuel power system isto provide a method of treating compressed crude carbon dioxide toremove nitrogen and argon and to reduce the concentration of oxygen toless than 100 ppm, preferably with low consumption of energy and highrecovery of carbon dioxide. Carbon dioxide recovery (based on carbondioxide in the total flue gas stream) should ideally be better than 97%.In addition, if the purified carbon dioxide product is produced as a lowtemperature liquid stream at a pressure below its critical pressure,transportation as a liquid or as a supercritical fluid to a carbondioxide storage site is facilitated.

A further method of carbon dioxide capture from fossil fuels is toconvert the fossil fuel to a mixture of carbon monoxide and hydrogencalled synthesis gas (or “syngas”) by catalytic reforming with steam; bypartial oxidation; by gas heated catalytic reforming; or by anycombination of these known processes, followed by shift reaction ofcarbon monoxide and water to produce a net hydrogen-rich product gascontaining carbon dioxide as the major impurity. These processes takeplace at high pressures, typically from about 20 bar to 70 bar.

Hydrogen must be separated from impurities such as methane and carbonmonoxide. Carbon monoxide must also be separated and purified. Apreferred method of purification is to use a multi-bed pressure swingadsorption (“PSA”) process to produce a pure hydrogen. A typical PSAunit, operating at 25 bar pressure, would have a typical recovery ofabout 85% to about 90% of hydrogen in the feed gas. The composition ofthe waste gas, typically at a pressure of about 1.2 bar to about 1.5bar, depends on the method used to produce the gas from the fossil fuel.For example, the PSA waste gas from a feed produced in a steam/naturalgas catalytic reformer would typically comprise at least about 60 mol %carbon dioxide, together with lower quantities of hydrogen, methane,carbon monoxide and water vapor. In this case, the objective would be toreduce the levels of carbon monoxide and methane to below 100 ppm.

FIG. 1 depicts a flow sheet for a prior art process for removal ofcontaminant gases from crude carbon dioxide produced in an oxyfuelcombustion process. The process is disclosed in “Carbon Dioxide Capturefor Storage in Deep Geological Formations—Results from the CO ₂ CaptureProject” (Capture and Separation of Carbon Dioxide from CombustionSources; Vol. 1; Chapter 26; pp 451-475; Elsevier).

In FIG. 1, the carbon dioxide separation is carried out in a lowtemperature processing plant which uses carbon dioxide refrigeration tocool the crude carbon dioxide feed gas down to a temperature withinabout 2° C. of the carbon dioxide freezing temperature. At this point, aphase separation of the uncondensed gas takes place and the gas phase,containing about 25 mol % carbon dioxide and about 75 mol % contaminantgases is separated, warmed and work expanded to produce power beforebeing vented to atmosphere.

The process separates the contaminant gases from the carbon dioxide at atemperature of −54.5° C. at a point close to the freezing temperature ofthe feed gas mixture, where the carbon dioxide vapor pressure is 7.4bar. The refrigeration duty is provided by evaporating two streams ofliquid carbon dioxide at pressure levels of 8.7 bar and 18.1 bar in heatexchangers E101 and E102. The two resultant carbon dioxide gas streamsare fed to the carbon dioxide compressors, K101 and K102, which usuallywill be stages of a multistage compressor.

In FIG. 1, a feed 130 of carbonaceous fuel is combusted with a feed 132of oxygen in an oxyfuel combustion unit R101 to produce a stream 134 offlue gas, the heat of which is used to generate steam in a powergeneration plant (not shown). Stream 134 is divided into a major part(stream 138) and a minor part (stream 136). Stream 138 is recycled tothe oxyfuel combustion unit R101. Stream 136 of flue gas is washed withwater in a gas-liquid contact vessel C105 to remove water solublecomponents and produce washed flue gas. A stream 142 of water is fed tothe vessel C105 and a stream 144 of water comprising water solublecomponents from the flue gas is removed therefrom to provide a stream146 of crude carbon dioxide gas (comprising about 73 mol % carbondioxide).

The stream 146 is compressed in compressor K105 to produce a stream 1 ofwashed flue gas at a pressure of about 30 bar, which is dried to adewpoint of less than −60° C. in a pair of thermally regenerateddesiccant driers C103 to produce a stream 2 of dried waste carbondioxide gas. Stream 2 is cooled by indirect heat exchange in the heatexchanger E101 to about −23° C. to produce a stream 3 of crude gaseouscarbon dioxide which is fed to a phase separation vessel C101 where itis separated to produce first carbon dioxide-enriched liquid and a firstvapor containing the majority of the contaminant gases.

A stream 4 of first carbon dioxide-enriched liquid is reduced inpressure in valve V101 to about 18 bar to produce a stream 5 of reducedpressure first carbon dioxide-enriched liquid which is vaporized byindirect heat exchange in heat exchanger E101 to provide refrigerationand to produce a stream 6 of first carbon dioxide-enriched gas.

A stream 7 of first vapor from phase separator C101 is cooled byindirect heat exchange in the heat exchanger E102 to −54.5° C. toproduce a stream 8 of partially condensed fluid which is fed to a secondphase separation vessel C102 where it is separated into second carbondioxide-enriched liquid and a second vapor, containing the majority ofthe remaining contaminant gases.

A stream 13 of second carbon dioxide-enriched liquid is warmed to atemperature of about −51° C. by indirect heat exchange in heat exchangerE102 to produce a stream 14 of warmed second carbon dioxide-enrichedliquid which is reduced in pressure to 8.7 bar in valve V102 to producea stream 15 of reduced pressure second carbon dioxide-enriched liquid.Stream 15 is vaporized and warmed by indirect heat exchange in the heatexchangers E101, E102 to provide refrigeration and produce a stream 16of second carbon dioxide-enriched gas. The initial warming of stream 13in heat exchanger E102 is critical to prevent freezing of the secondcarbon dioxide enriched liquid on pressure reduction from about 30 bar.

A stream 9 of the second vapor from phase separator C102 is heated byindirect heat exchange to ambient temperature in the heat exchangersE101, E102 to produce a stream 10 of warmed second gas which is heatedby indirect heat exchange in pre-heater E103 to about 300° C. to producea stream 11 of pre-heated second gas. Stream 11 is work expanded inturbine K103 to produce power and a stream 12 of waste gas comprisingabout 25 mol % carbon dioxide and most of the contaminant gases which isthen vented the atmosphere.

Stream 16 is compressed in the first stage K102 of a multi-stagecentrifugal carbon dioxide compressor to produce a stream 17 ofcompressed carbon dioxide gas at a pressure of about 18 bar. Heat ofcompression is removed from stream 17 in an intercooler E104 usingcooling water as the coolant. A stream 18 of cooled compressed carbondioxide gas is combined with stream 6 and the combined stream is furthercompressed in the second or further stage(s) K101 of the compressor toproduce a stream 19 of further compressed carbon dioxide gas at apressure of about 110 bar. The concentration of carbon dioxide in stream19 is about 96 mol %. Heat of compression is removed from stream 19 inan aftercooler E105 using boiler feed water and/or condensate as acoolant thereby heating the boiler feed water and/or condensate andproducing a stream 20 of cooled further compressed carbon dioxide gas atpipeline pressure, e.g. at about 110 bar.

For simplicity, heat exchangers E101 and E102 are shown in FIG. 1 asseparate heat exchangers. However, as would be appreciated by theskilled person, heat exchangers E101 and E102 would usually, in reality,form parts of the main heat exchanger with feed streams entering andproduct streams leaving at the most thermodynamically efficientlocations. The main heat exchanger E101, E102 is usually a multi-streamplate-fin heat exchanger, preferably made from aluminum.

Table 1 is a heat and mass balance table for the process depicted inFIG. 1.

TABLE 1 Stream Number 1 2 3 4 5 6 7 8 9 10 Tempera- ° C. 24.83 24.83−22.66 −22.66 −30.87 11.21 −22.66 −54.50 −54.50 11.21 ture Pressure bara 30 30 29.8 29.8 18.12636 18.02636 29.8 29.7 29.7 29.65 Flow kg/s140.49 140.40 140.40 27.73 27.73 27.73 112.67 112.67 37.75 37.75Composi- tion CO2 mol % 72.7633 72.8651 72.8651 97.6055 97.6055 97.605567.3695 67.3695 24.7546 24.7546 N2 mol % 18.9694 18.9959 18.9959 1.50141.5014 1.5014 22.8819 22.8819 53.4392 53.4392 Ar mol % 2.6956 2.69942.6994 0.3712 0.3712 0.3712 3.2165 3.2165 6.9090 6.9090 O2 mol % 5.43165.4392 5.4392 0.5218 0.5218 0.5218 6.5314 6.5314 14.8960 14.8960 H2O mol% 0.1396 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000SO2 ppm 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 NO ppm 4.9674 4.9743 4.9743 0.6929 0.6929 0.6929 5.9254 5.925412.0859 12.0859 NO2 ppm 0.0043 0.0043 0.0043 0.0210 0.0210 0.0210 0.00060.0006 0.0000 0.0000 Stream Number 11 12 13 14 15 16 17 18 19 20Tempera- ° C. 300.00 20.07 −54.50 −42.85 −55.50 11.21 69.17 25.00 195.1043.00 ture Pressure bar a 29.65 1.1 29.7 29.65 8.743321 8.54332118.12636 18.02636 110 110 Flow kg/s 37.75 37.75 74.92 74.92 74.92 74.9274.92 74.92 102.65 102.65 Composi- tion CO2 mol % 24.7546 24.754695.2747 95.2747 95.2747 95.2747 95.2747 95.2747 95.9012 95.9012 N2 mol %53.4392 53.4392 2.8723 2.8723 2.8723 2.8723 2.8723 2.8723 2.5038 2.5038Ar mol % 6.9090 6.9090 0.7986 0.7986 0.7986 0.7986 0.7986 0.7986 0.68370.6837 O2 mol % 14.8960 14.8960 1.0542 1.0542 1.0542 1.0542 1.05421.0542 0.9111 0.9111 H2O mol % 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 0.0000 SO2 ppm 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 0.0000 0.0000 NO ppm 12.0859 12.0859 1.8913 1.89131.8913 1.8913 1.8913 1.8913 1.5692 1.5692 NO2 ppm 0.0000 0.0000 0.00100.0010 0.0010 0.0010 0.0010 0.0010 0.0063 0.0063 **The process depictedin FIG. 1 produces purified carbon dioxide having a carbon dioxideconcentration of about 96 mol % and containing about 0.9 mol % oxygen ata carbon dioxide recovery of about 89%.

The general concept of using distillation to purify carbon dioxideproduced in an oxyfuel combustion process is not new. In thisconnection, Allam et al (“A Study of the Extraction of CO ₂ from theFlue Gas of a 500 MW Pulverized Coal Fired Boiler”, Allam and Spilsbury;Energy Consers. Mgmt; Vol. 33; No. 5-8, pp 373-378; 1992) discloses aprocess for purifying carbon dioxide from an oxyfuel combustion processusing distillation to purify the carbon dioxide to remove “heavy”impurities (such as sulfur dioxide and nitrogen dioxide), andcontaminant gases including oxygen, nitrogen and argon.

In Allam et al, the carbon dioxide system is integrated with an airseparation unit (“ASU”), using expansion of both the nitrogen and oxygenstreams to provide refrigeration for the carbon dioxide liquefactionprocess. The process recycles part of the oxygen-containing streamseparated from the carbon dioxide to the boiler, taking a purge streamat this point to prevent contaminants build up. A rectifying column isused at the cold end to remove lighter contaminants from the carbondioxide stream. A second column, also at the cold end, removes sulfurdioxide and nitrogen oxides from the resultant carbon dioxide stream.

In addition, the general idea that a distillation column could be usedto remove oxygen from carbon dioxide produced oxyfuel combustion processwas disclosed by the Inventors in a paper entitled “Purification ofOxyfuel-Derived CO ₂ for Sequestration or EOR” presented at the 8^(th)Greenhouse Gas Control Technologies conference (GHGT-8), Trondheim, inJune 2006. However, no details regarding how the general idea might beimplemented were disclosed.

Other prior art includes GB-A-2151597 (Duckett; published 1985) whichdescribes a process of using membranes to concentrate a lowconcentration carbon dioxide feed stream so that it can be purifiedusing phase separation. The aim is to make liquid carbon dioxide forsale rather than to recover as much carbon dioxide as possible from acombustion process and, accordingly, carbon dioxide recovery from thefeed is very low at about 70%.

GB-A-2151597 discloses the use of the carbon dioxide feed stream toprovide heat to the reboiler of the distillation column. GB-A-2151597also discloses the use of an external refrigeration source to providethe liquid required for the distillation process to work.

U.S. Pat. No. 4,602,477 (Lucadamo; published July 1986) discloses aprocess for taking hydrocarbon offgas and increasing its value byseparating it into a light hydrocarbon stream, a heavy hydrocarbonstream, and a waste carbon dioxide stream. The presence of the carbondioxide in the stream decreases the heating and economic value of thegas. The process uses a carbon dioxide membrane unit to perform a finalremoval of carbon dioxide from the light hydrocarbon product, inaddition to a distillation step performed at low temperatures.

The aim of the process disclosed in U.S. Pat. No. 4,602,477 is not toproduce high purity carbon dioxide but to remove carbon dioxide from thehydrocarbon feed. The distillation step produces the carbon dioxidestream as a side stream from a rectifying column having a condenser. Theprocess also uses a stripping column to purify the heavy hydrocarbonstream.

U.S. Pat. No. 4,977,745 (Heichberger; published in December 1990)discloses a process for purifying a feed stream having a carbon dioxidefeed purity of greater than 85 mol %. The high pressure residual streamis heated and expanded to recover power but an external refrigerationsource is used to liquefy the carbon dioxide.

EP-A-0964215 (Novakand et al; published in December 1999) discloses therecovery of carbon dioxide from a process using carbon dioxide to freezefood. The process involves the use of a distillation column to recoverthe carbon dioxide. The carbon dioxide feed stream to the columnprovides reboiler duty to the column before being fed to the column asreflux.

U.S. Pat. No. 4,952,223 (Kirshnamurthy et al; published in August 1990)discloses a carbon dioxide liquefaction process in which the carbondioxide recovery is improved by passing the vent gas to a PSA system toproduce a carbon dioxide-enriched recycle stream and a carbondioxide-depleted vent stream.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodfor removing a first contaminant selected from oxygen and carbonmonoxide from impure liquid carbon dioxide, said method comprising:

-   -   separating said impure liquid carbon dioxide in a mass transfer        separation column system to produce first contaminant-enriched        overhead vapor and carbon dioxide-enriched bottoms liquid; and    -   reboiling a portion of said carbon dioxide-enriched bottoms        liquid by indirect heat exchange against crude carbon dioxide        fluid to produce carbon dioxide-enriched vapor for said column        system and cooled crude carbon dioxide fluid;        wherein said impure liquid carbon dioxide has a greater        concentration of carbon dioxide than said crude gaseous carbon        dioxide.

The invention has particular application in a method for recoveringcarbon dioxide from contaminated carbon dioxide gas comprising a firstcontaminant selected from the group consisting of oxygen and carbonmonoxide, and at least about 60 mol % carbon dioxide, said methodcomprising:

-   -   combining at least a portion of contaminated carbon dioxide gas        feed with compressed first contaminant-enriched gas recycled        from downstream to produce crude carbon dioxide gas;    -   cooling at least a portion of said crude carbon dioxide gas by        indirect heat exchange, usually with at least one process        stream, to produce crude carbon dioxide fluid;    -   separating impure liquid carbon dioxide comprising said first        contaminant in a mass transfer separation column system to        produce first contaminant-enriched overhead vapor and carbon        dioxide-enriched bottoms liquid;    -   reboiling a portion of said carbon dioxide-enriched bottoms        liquid by indirect heat exchange against at least a portion of        said crude carbon dioxide fluid to produce carbon        dioxide-enriched vapor for said column system and cooled crude        carbon dioxide fluid;    -   further cooling at least a portion of said cooled crude carbon        dioxide fluid by indirect heat exchange, usually with at least        one process stream, to produce partially condensed crude carbon        dioxide fluid;    -   phase separating at least a portion of said partially condensed        crude carbon dioxide fluid to produce said impure liquid carbon        dioxide and carbon dioxide-depleted vapor;    -   feeding at least a portion of said impure liquid carbon dioxide        to said column system for separation;    -   dividing a portion of said carbon dioxide-enriched bottoms        liquid into a first part and at least one further part;    -   expanding said first part to produce an expanded first part at a        first pressure;    -   vaporizing said expanded first part by indirect heat exchange,        usually with at least one process stream, to provide a portion        of the refrigeration duty required by the method and produce        carbon dioxide gas;    -   expanding the at least one further part to produce at least one        expanded further part having a pressure that is higher than said        first pressure;    -   vaporizing the or each expanded further part by indirect heat        exchange, usually with at least one process stream, to provide        at least a portion of the remaining refrigeration duty required        by the method and produce carbon dioxide gas;    -   warming at least a portion of said first contaminant-enriched        overhead vapor by indirect heat exchange, usually with at least        one process stream, to produce warmed first contaminant-enriched        gas;    -   compressing at least a portion of said warmed first        contaminant-enriched gas to produce said compressed first        contaminant-enriched gas for recycling to said contaminated        carbon dioxide gas feed; and    -   compressing said carbon dioxide gases to form compressed carbon        dioxide gas.

According to a second aspect of the present invention, there is providedapparatus for carrying out the method of the first aspect, saidapparatus comprising:

-   -   a mass transfer separation column system for separating said        impure liquid carbon dioxide to produce first        contaminant-enriched overhead vapor and carbon dioxide-enriched        bottoms liquid;    -   a reboiler for re-boiling carbon dioxide-enriched bottoms liquid        by indirect heat exchange against crude carbon dioxide fluid to        produce carbon dioxide-enriched vapor for said column system and        cooled crude carbon dioxide fluid;    -   a heat exchanger for further cooling cooled crude carbon dioxide        fluid by indirect heat exchange, usually with at least one        process stream, to produce partially condensed crude carbon        dioxide fluid;    -   a conduit arrangement for feeding cooled crude carbon dioxide        fluid from said reboiler to said heat exchanger;    -   a phase separator for phase separating said partially condensed        crude carbon dioxide fluid to produce said impure liquid carbon        dioxide and carbon dioxide-depleted vapor;    -   a conduit arrangement for feeding partially condensed crude        carbon dioxide fluid from said heat exchanger to said phase        separator;    -   a first pressure reduction arrangement for reducing the pressure        of impure liquid carbon dioxide to produce reduced pressure        impure liquid carbon dioxide;    -   a conduit arrangement for feeding impure liquid carbon dioxide        from said phase separator to said first pressure reduction        arrangement; and    -   a conduit arrangement for feeding reduced pressure impure liquid        carbon dioxide from said first pressure reduction arrangement to        said column system.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic representation (flow sheet) of a prior art processfor recovering carbon dioxide from flue gas generated in an oxyfuelcombustion process;

FIG. 2 is a schematic representation (flow sheet) of embodiments of thepresent invention in which refrigeration duty is provided by two streamsof expanded carbon dioxide-enriched liquid; and

FIG. 3 is a schematic representation (flow sheet) of embodiments of thepresent invention in which refrigeration duty is provided by threestreams of expanded carbon dioxide-enriched liquid.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the present invention comprises separating saidimpure liquid carbon dioxide in a mass transfer separation column systemto produce first contaminant-enriched overhead vapor and carbondioxide-enriched bottoms liquid and reboiling a portion of the carbondioxide-enriched bottoms liquid by indirect heat exchange against crudecarbon dioxide fluid to produce carbon dioxide-enriched vapor for thecolumn system and cooled crude carbon dioxide fluid. The method ischaracterized in that the impure liquid carbon dioxide has a greaterconcentration of carbon dioxide than the crude carbon dioxide fluid.

Other contaminants are usually present in the impure liquid carbondioxide. For example, if the method is used to recover carbon dioxidefrom flue gas produced in an oxyfuel combustion process, the othercontaminants usually include oxygen, nitrogen and argon; oxides ofsulfur (e.g. sulfur dioxide); and oxides of nitrogen (e.g. nitric oxideand nitrogen dioxide). If the method is used to recover carbon dioxidefrom off gas produced in a hydrogen PSA process, other contaminantsusually include hydrogen; carbon monoxide; nitrogen; methane; and argon.The method of the present invention preferably also removes the bulk ofthese other contaminants from the impure liquid carbon dioxide.

The crude gaseous carbon dioxide typically comprises at least about 60mol % carbon dioxide, and usually comprises no more than 90 mol % carbondioxide. In preferred embodiments, the crude gaseous carbon dioxidecomprises from at least about 65 mol % to about 90 mol %, carbondioxide, e.g. from about 70 mol % to about 75 mol %.

The impure liquid carbon dioxide typically comprises at least about 90mol %, and usually comprises no more than about 99 mol %, carbondioxide. In preferred embodiments, the impure liquid carbon dioxidecomprises from about 95 mol % to about 99 mol % carbon dioxide.

In preferred embodiments, the impure carbon dioxide liquid is derivedfrom the cooled crude carbon dioxide fluid. In such embodiments, themethod may further comprise:

-   -   further cooling at least a portion of said cooled crude carbon        dioxide fluid by indirect heat exchange, usually with at least        one process stream, to produce partially condensed crude carbon        dioxide fluid; and    -   phase separating at least a portion of said partially condensed        crude carbon dioxide fluid to produce said impure liquid carbon        dioxide and carbon dioxide-depleted vapor.

The operating pressure(s) of the column system is usually lower than thepressure of the impure liquid carbon dioxide. Thus, in theseembodiments, the pressure of the impure liquid carbon dioxide ispreferably reduced to about the operating pressure of the column systemwithout forming solid carbon dioxide prior to feeding the impure liquidcarbon dioxide to the column system.

Avoiding formation of solid carbon dioxide during pressure reduction maybe achieved by warming the impure liquid carbon dioxide by indirect heatexchange, usually with at least one process stream, prior to reducingthe pressure thereof. For example, in the exemplified embodiments, theimpure liquid carbon dioxide is warmed to about −30° C.

At least a portion of the entire refrigeration duty required by themethod of the present invention is usually provided by vaporizing aportion of the carbon dioxide-enriched bottoms liquid by indirect heatexchange with at least one process stream, preferably after expansion.

The method usually comprises expanding at least a first part of thecarbon dioxide-enriched liquid to produce an expanded first part at afirst pressure; and vaporizing the expanded first part by indirect heatexchange, usually with at least one process stream, to provide a portionof the refrigeration duty required by the method and produce carbondioxide gas.

The first pressure is usually from about the triple point pressure forcarbon dioxide, i.e. 5.18 bar, to about 15 bar, and is preferably nomore than about 6 bar.

The method preferably comprises:

-   -   expanding at least one further part of said carbon        dioxide-enriched bottoms liquid to produce at least one expanded        further part having a pressure that is higher than said first        pressure;    -   vaporizing at least a portion of the at least one expanded        further part by indirect heat exchange, usually with at least        one process stream, to provide at least a portion of the        remaining refrigeration duty required by the method and produce        carbon dioxide gas. For example, the at least one expanded        further part may be used to provide at least a portion of the        refrigeration duty required to cool crude carbon dioxide gas to        produce the crude carbon dioxide fluid.

The pressure(s) of the at least one expanded further part is usuallyfrom about the triple point pressure for carbon dioxide to about 20 bar.In some embodiments, there is only one further part which is expanded toa second pressure which is usually from about the triple point pressurefor carbon dioxide to about 20 bar, preferably from about 12 bar toabout 18 bar, e.g. about 15 bar. In other embodiments, there are twofurther parts, one part being expanded to the second pressure and theother part being expanded to a third pressure which is higher then thefirst pressure and lower than the second pressure. The third pressure isusually from about the triple point pressure for carbon dioxide to about20 bar, preferably from about 8 bar to about 14 bar, e.g. about 10 bar.

In preferred embodiments, the majority, i.e. over 50%, of the entirerefrigeration duty required by the method of the present invention isprovided by vaporization of carbon dioxide-enriched bottoms liquid,usually after suitable pressure reduction(s). Preferably, at least 75%and, most preferably, at least 90% of the entire refrigeration duty isprovided by such vaporization.

Any remaining refrigeration duty not provided by vaporization of carbondioxide-enriched bottoms liquid may be provided by vaporizing anexternal refrigerant. However, it is preferred that the entirerefrigeration duty required by the method is provided internally, i.e.without the use of an external refrigerant, by indirect heat exchangebetween process streams.

The expression “refrigeration duty” refers only to the sub-ambientrefrigeration duty, i.e. the refrigeration duty below ambienttemperature, and excludes cooling duty at a temperature at or aboveambient temperature.

The carbon dioxide gas(es) produced by indirect heat exchange against atleast one process stream after providing refrigeration may be compressedin a carbon dioxide compression train to pipe line pressure, e.g. fromabout 100 bar to about 250 bar.

At least a portion of the carbon dioxide-depleted vapor is usuallywarmed by indirect heat exchange with at least one process stream, e.g.to ambient temperature, to produce carbon dioxide-depleted gas. At leasta portion of the carbon dioxide-depleted gas may be heated by indirectheat exchange and then work expanded to produce power and expandedcarbon dioxide-depleted gas which is usually vented to the atmosphere.Typically, all of the contaminants are eventually vented in the expandedcarbon dioxide-depleted gas due to the recycle of the firstcontaminant-enriched gas.

In preferred embodiments, the method comprises:

-   -   warming at least a portion of the carbon dioxide-depleted vapor        by indirect heat exchange, usually with at least one process        stream, to produce carbon dioxide-depleted gas;    -   pre-heating at least a portion of the carbon dioxide-depleted        gas by indirect heat exchange to produce pre-heated carbon        dioxide-depleted gas; and    -   work expanding at least a portion of the pre-heated carbon        dioxide-depleted gas to produce expanded carbon dioxide-depleted        gas;        wherein at least a portion of the heat required to pre-heat the        carbon dioxide-depleted gas is provided by recovering heat of        compression from contaminated carbon dioxide gas.

In preferred embodiments, the impure liquid carbon dioxide is fed to thecolumn system at a location at or near the top of the or each column.

Preferred embodiments of the method comprise:

-   -   warming at least a portion of said first contaminant-enriched        overhead vapor by indirect heat exchange, usually with at least        one process stream, to produce warmed first contaminant-enriched        gas;    -   compressing at least a portion of said warmed first        contaminant-enriched gas to produce compressed first        contaminant-enriched gas;    -   combining at least a portion of said compressed first        contaminant-enriched gas with a contaminated carbon dioxide feed        gas to form said crude carbon dioxide gas; and    -   cooling at least a portion of said crude carbon dioxide gas by        indirect heat exchange, usually with at least one process        stream, prior to providing said reboil to the column system. At        least a portion of the heat of compression from the compressed        first contaminant-enriched gas may be removed by indirect heat        exchange with a coolant, preferably water, prior to combining        with contaminated carbon dioxide gas.

The method may be applied to recover carbon dioxide from any stream ofwaste gas comprising at least about 60 mol % carbon dioxide. However,the method has particular application in the recovery of carbon dioxidefrom flue gas generated in an oxyfuel combustion process or waste gasfrom a hydrogen PSA process.

In some embodiments, the first contaminant is oxygen. In theseembodiments, the impure liquid carbon dioxide may be produced from fluegas generated in an oxyfuel combustion process.

Flue gas from an oxyfuel combustion process is usually generated bycombusting a fuel selected from the group consisting of carbonaceousfuel; hydrocarbonaceous fuel; and mixtures thereof, in the presence ofpure oxygen. The flue gas is usually washed with water to remove atleast the majority of water soluble contaminants and to cool the gas.The resultant washed flue gas is usually compressed to form compressedflue gas which is then usually then dried to form at least part of thecrude carbon dioxide gas.

The washing step usually takes place in a counter current gas-liquidcontact vessel such as a wash (or scrub) column.

The washed flue is compressed to the operating pressure of the gasdrying system. In embodiments in which the gas drying system is at leastone desiccant drier, the operating pressure is usually about 10 bar toabout 50 bar, and preferably from about 25 bar to about 35 bar, e.g.about 30 bar. Heat of compression may be recovered from compressed fluegas to pre-heat carbon dioxide-depleted gas before work expansion andventing.

The method disclosed in U.S. Ser. No. 11/287,640 (the disclosure ofwhich has been incorporated herein by reference) may be integrated withthe method of the present invention to remove at least a portion of oneor more further contaminants selected from the group consisting ofsulfur dioxide and NO_(x) (i.e. nitric oxide and nitrogen dioxide) fromthe carbon dioxide gas in the carbon dioxide compression train. In thisconnection, the method of the present invention may further comprise:

-   -   compressing flue gas, or a gas derived therefrom, to an elevated        pressure(s), usually from about 10 bar to about 50 bar;    -   maintaining said flue gas at said elevated pressure in the        presence of oxygen and water and, when sulfur dioxide is to be        removed, NO_(x), for a sufficient time to covert sulfur dioxide        to sulfuric acid and/or NO_(x) to nitric acid; and    -   separating the sulfuric acid and/or nitric acid from the flue        gas to produce sulfur dioxide-free, NO_(x)-lean crude carbon        dioxide gas which is usually then fed to the gas drying system        after further compression to the operating pressure thereof if        necessary. One advantage of these embodiments is that any        mercury present in the carbon-dioxide enriched gas is also        removed.

Where the crude carbon dioxide gas comprises SO₂ and NO_(x), the methodpreferably comprises converting SO₂ to sulfuric acid at a first elevatedpressure and converting NO_(x) to nitric acid at a second elevatedpressure which is higher than the first elevated pressure. A portion ofthe NO_(x) may be converted to nitric acid at the first elevatedpressure. For example, if SO₂ feed concentration is sufficiently low,there could be more nitric acid than sulfuric acid produced at the firstelevated pressure.

In these embodiments, the method usually comprises:

-   -   washing flue gas, or a gas derived therefrom, with water at said        first elevated pressure in a first counter current gas/liquid        contact device to produce SO₂-free carbon dioxide gas and an        aqueous sulfuric acid solution;    -   compressing at least a portion of the SO₂-free carbon dioxide        gas to the second elevated pressure; and    -   washing at least a portion of the SO₂-free carbon dioxide gas        with water at the second elevated pressure in a second counter        current gas/liquid contact device to produce SO₂-free,        NO_(x)-lean carbon dioxide gas and an aqueous nitric acid        solution. At least a portion of the SO₂-free, NO_(x)-lean carbon        dioxide gas is then fed, after optional further compression if        necessary, to the gas drying system for drying to produce said        contaminated carbon dioxide gas.

At least a portion of the aqueous sulfuric acid solution is usuallyrecycled to the first gas/liquid contact device, optionally afterpumping and/or cooling. At least a portion of the aqueous nitric acidsolution is usually recycled to the second gas/liquid contact device,optionally after pumping and/or cooling.

The first elevated pressure is usually from 10 bar to 20 bar and ispreferably about 15 bar. Where the gaseous carbon dioxide is compressedto the first elevated pressure, such compression is preferablyadiabatic. The second elevated pressure is usually from 25 bar to 35 barand is preferably about 30 bar.

Embodiments of the present method in which the first contaminant isoxygen may be incorporated into the method disclosed in the sisterapplication, U.S. Ser. No. 11/656,912, (to be advised), and filed on thesame day as the present application, the disclosure of which isincorporated herein by reference. In this connection, the method of thepresent invention may comprise:

-   -   combusting a fuel selected from carbonaceous fuel;        hydrocarbonaceous fuel; and mixtures thereof, in the presence of        oxygen in an oxyfuel combustion unit to produce flue gas        comprising carbon dioxide;    -   warming at least a portion of the carbon dioxide-depleted vapor        by indirect heat exchange, usually with at least one process        stream, to produce carbon dioxide-depleted gas;    -   separating carbon dioxide from at least a portion of the carbon        dioxide-depleted gas by diffusion across at least one permeable        membrane in a membrane separation system to produce separated        carbon dioxide gas and vent gas; and    -   feeding at least a portion of the separated carbon dioxide gas        from the membrane separation system to the oxyfuel combustion        unit to reduce the temperature of combustion. The vent gas may        be work expanded to produce power and then vented to the        atmosphere.

In other embodiments, the first contaminant is carbon monoxide. In theseembodiments, the impure liquid carbon dioxide may be produced from wastegas from a hydrogen PSA process.

Carbonaceous fuel (e.g. coal) or hydrocarbonaceous fuel (e.g. methane ornatural gas) may be converted to syngas by catalytic reforming withsteam; partial oxidation; gas heated catalytic reforming; or anycombination of these processes. Syngas may be subjected to shiftreaction with water to produce hydrogen-enriched gas comprising carbondioxide as a major component. These processes typically take place at apressure from about 20 bar to about 70 bar.

Hydrogen may be separated from the hydrogen-enriched gas by a PSAsystem, usually a multi-bed PSA unit. A PSA system typically operates atabout 25 bar. The composition of the waste gas stream from the PSAsystem depends on the fuel used but would usually comprise at leastabout 60 mol % carbon dioxide with lower quantities of hydrogen,methane, carbon monoxide and water.

The mass transfer separation column system usually comprises a singledistillation (or stripping) column. The column is usually operated at apressure that is lower than the pressure of the crude carbon dioxidefluid. In this connection, the operating pressure of the column isusually from about 5 bar to about 50 bar and, preferably, from about 14bar to about 18 bar, e.g. about 16 bar. The pressure of the crude carbondioxide fluid is usually from about 15 bar to about 60 bar and,preferably, from about 25 bar to about 35 bar, e.g. about 30 bar.

The apparatus comprises:

-   -   a mass transfer separation column system for separating impure        liquid carbon dioxide to produce first contaminant-enriched        overhead vapor and carbon dioxide-enriched bottoms liquid;    -   a reboiler for re-boiling carbon dioxide-enriched bottoms liquid        by indirect heat exchange against crude carbon dioxide fluid to        produce carbon dioxide-enriched vapor for the column system and        cooled crude carbon dioxide fluid;    -   a heat exchanger for further cooling cooled crude carbon dioxide        fluid by indirect heat exchange, usually with at least one        process stream, to produce partially condensed crude carbon        dioxide fluid;    -   a conduit arrangement for feeding cooled crude carbon dioxide        fluid from the reboiler to the heat exchanger;    -   a phase separator for phase separating the partially condensed        crude carbon dioxide fluid to produce the impure liquid carbon        dioxide and carbon dioxide-depleted vapor;    -   a conduit arrangement for feeding partially condensed crude        carbon dioxide fluid from the heat exchanger to the phase        separator;    -   a first pressure reduction arrangement for reducing the pressure        of impure liquid carbon dioxide to produce reduced pressure        impure liquid carbon dioxide;    -   a conduit arrangement for feeding impure liquid carbon dioxide        from the phase separator to the first pressure reduction        arrangement; and    -   a conduit arrangement for feeding reduced pressure impure liquid        carbon dioxide from the first pressure reduction arrangement to        the column system. The reboiler may be located either within the        column system (e.g. in the sump of the column) or outside the        column system connected by suitable conduit arrangement(s) as is        known in the art.

An “arrangement” for carrying out a particular function is a device ordevices adapted and constructed to carry out that function. In thisconnection, a “conduit arrangement” is any form of conduit suitable fortransferring the relevant fluid between the indicated parts of theapparatus. An example of a suitable conduit arrangement is at least onepipe or pipework. However, a “conduit arrangement” may also compriseother apparatus where appropriate. For example, the conduit arrangementfor feeding impure liquid carbon dioxide from the phase separator to thefirst pressure reduction arrangement may comprise:

-   -   a conduit arrangement for feeding impure liquid carbon dioxide        from the phase separator to the heat exchanger for warming to        provide warmed impure liquid carbon dioxide;    -   at least one fluid passage in the heat exchanger; and    -   a conduit arrangement for feeding warmed impure liquid carbon        dioxide from the heat exchanger to the first pressure reduction        arrangement.

The apparatus preferably comprises:

-   -   a second pressure reduction arrangement for expanding carbon        dioxide-enriched bottoms liquid to produce expanded carbon        dioxide-enriched bottoms liquid at a first pressure;    -   a conduit arrangement for feeding carbon dioxide-enriched        bottoms liquid from the column system to the second pressure        reduction arrangement; and    -   a conduit arrangement for feeding expanded carbon        dioxide-enriched bottoms liquid at the first pressure from the        second pressure reduction arrangement to the heat exchanger for        vaporization to provide refrigeration duty.

In preferred embodiments, the apparatus comprises:

-   -   a third pressure reduction arrangement for expanding carbon        dioxide-enriched bottoms liquid to produce expanded carbon        dioxide-enriched bottoms liquid at a second pressure which is        higher than the first pressure;    -   a conduit arrangement for feeding carbon dioxide-enriched        bottoms liquid from column system to the third pressure        reduction arrangement; and    -   a conduit arrangement for feeding expanded carbon        dioxide-enriched bottoms liquid at the second pressure from the        third pressure reduction arrangement to the heat exchanger for        vaporization to provide refrigeration duty. The conduit        arrangement for feeding carbon dioxide-enriched bottoms liquid        may feed said liquid directly from the column system or from        another conduit arrangement carrying said fluid.

In certain preferred embodiments, the apparatus preferably comprises:

-   -   a fourth pressure reduction arrangement for expanding carbon        dioxide-enriched bottoms liquid to produce expanded carbon        dioxide-enriched bottoms liquid at a third pressure which is        higher than the first pressure and lower than the second        pressure;    -   a conduit arrangement for feeding carbon dioxide-enriched        bottoms liquid from column system to the fourth pressure        reduction arrangement; and    -   a conduit arrangement for feeding the expanded carbon        dioxide-enriched bottoms liquid at the third pressure from the        fourth pressure reduction arrangement to the heat exchanger for        vaporization to provide refrigeration duty. The conduit        arrangement for feeding carbon dioxide-enriched bottoms liquid        from column system to the fourth pressure reduction arrangement        may feed carbon dioxide-enriched bottoms liquid directly from        the column system or from another conduit arrangement carrying        said fluid.

The apparatus preferably comprises:

-   -   a conduit arrangement for feeding first contaminant-enriched        overhead vapor from the column system to the heat exchanger for        warming to provide warmed first contaminant-enriched gas;    -   a recycle compressor arrangement for compressing warmed first        contaminant-enriched gas to produce compressed first        contaminant-enriched gas;    -   a conduit arrangement for feeding warmed first        contaminant-enriched gas from the heat exchanger to the recycle        compressor arrangement;    -   a conduit arrangement for combining compressed first        contaminant-enriched gas from the recycle compressor arrangement        with contaminated carbon dioxide gas to form crude carbon        dioxide gas;    -   a conduit arrangement for feeding the crude carbon dioxide gas        from the conduit arrangement combining said contaminated gases        to the heat exchanger for cooling to provide crude carbon        dioxide fluid; and    -   a conduit arrangement for feeding crude carbon dioxide fluid        from the heat exchanger to the reboiler.

The “recycle compressor arrangement” is typically a single stagecompressor, usually with an aftercooler. Thus, the conduit arrangementfor combining the contaminated gases may comprise:

-   -   an aftercooler for removing heat of compression from compressed        first contaminant-enriched gas by indirect heat exchange with a        coolant, usually water, to produce cooled compressed first        contaminant-enriched gas;    -   a conduit arrangement for feeding compressed first        contaminant-enriched gas from the recycle compressor arrangement        to the aftercooler;    -   a conduit arrangement for combining cooled compressed first        contaminant-enriched gas from the aftercooler with the        contaminated carbon dioxide gas

In embodiments in which the contaminated carbon dioxide gas is derivedfrom flue gas produced in an oxyfuel combustion process, the apparatusmay comprise:

-   -   an oxyfuel combustion unit for combusting a fuel selected from        the group consisting of carbonaceous fuel; hydrocarbonaceous        fuel; and mixtures thereof, in the presence of oxygen to produce        flue gas comprising carbon dioxide;    -   a conduit arrangement for recycling a portion of the flue gas to        the oxyfuel combustion unit;    -   a gas-liquid contact vessel for washing at least a part of the        remaining portion of the flue gas with water to remove water        soluble components and produce washed flue gas;    -   a conduit arrangement for feeding flue gas from the oxyfuel        combustion unit to the gas-liquid contact vessel;    -   a flue gas compressor arrangement for compressing washed flue        gas to produce compressed flue gas;    -   a conduit arrangement for feeding washed flue gas from the        gas-liquid contact vessel to the flue gas compressor        arrangement;    -   a gas drying system for drying compressed flue gas to produce        contaminated carbon dioxide gas;    -   a conduit arrangement for feeding compressed flue gas from the        flue gas compressor arrangement to the gas drying system; and    -   a conduit arrangement for feeding contaminated carbon dioxide        gas, or a gas derived therefrom, to the reboiler.

The “flue gas compression arrangement” is usually a single stage ormultiple stage centrifugal compressor or is one or more stages of amultiple stage centrifugal compressor with optional intercooling.

In embodiments where the first contaminant is oxygen, the apparatus maycomprise:

-   -   a conduit arrangement for feeding carbon dioxide-depleted vapor        from the phase separator to the heat exchanger for warming to        produce carbon dioxide-depleted gas;    -   a membrane separation system comprising at least one permeable        membrane for separating carbon dioxide from carbon        dioxide-depleted gas by diffusion across said membrane(s) to        produce separated carbon dioxide gas and vent gas;    -   a conduit arrangement for feeding carbon dioxide-depleted gas        from the heat exchanger to the membrane separation system;    -   an oxyfuel combustion unit for combusting a fuel selected from        the group consisting of carbonaceous fuel; hydrocarbonaceous        fuel; and mixtures thereof, in the presence of oxygen to produce        flue gas comprising carbon dioxide; and    -   a conduit arrangement for feeding separated carbon dioxide gas        from the membrane separation system to the oxyfuel combustion        unit.

In embodiments in which the waste carbon dioxide gas is flue gasproduced in an oxyfuel combustion process, the apparatus usuallycomprises:

-   -   a gas-liquid contact vessel for washing at least a portion of        said flue gas with water to remove water soluble components and        produce washed flue gas;    -   a conduit arrangement for feeding flue gas from the oxyfuel        combustion unit to the gas-liquid contact vessel;    -   a first compressor arrangement for compressing washed flue gas        to produce compressed flue gas;    -   a conduit arrangement for feeding washed flue gas from the        gas-liquid contact vessel to the first compressor arrangement;    -   a gas drying system for drying compressed flue gas to produce        contaminated carbon dioxide gas;    -   a conduit arrangement for feeding compressed flue gas from said        first compressor arrangement to said gas drying system; and    -   a conduit arrangement for feeding contaminated carbon dioxide        gas, or a gas derived therefrom, to the heat exchanger.

In embodiments including the removal of one or more contaminantsselected from the group consisting of SO₂ and NO_(x) from crude carbondioxide gas, said apparatus may comprise:

-   -   at least one counter current gas/liquid contact device for        washing flue gas with water at elevated pressure in the presence        of oxygen and, when SO₂ is to be removed, NO_(x), for a        sufficient time to convert SO₂ to sulfuric acid and/or NO_(x) to        nitric acid;    -   a conduit arrangement for feeding flue gas at elevated pressure        from said first compressor arrangement to the or each respective        gas/liquid contact device; and    -   a conduit arrangement(s) for recycling aqueous sulfuric acid        solution and/or aqueous nitric acid solution to the or each        respective gas/liquid contact device.

In embodiments where the first compressor arrangement is a multi-stagecompressor, the apparatus may comprise:

-   -   a first compressor for compressing flue gas, or a gas derived        therefrom, to a first elevated pressure;    -   a conduit arrangement for feeding flue gas, or a gas derived        therefrom, to said first compressor;    -   a first counter current gas/liquid contact device for washing        compressed flue gas with water at the first elevated pressure        for a sufficient time to produce SO₂-free carbon dioxide gas and        an aqueous sulfuric acid solution;    -   a conduit arrangement for feeding compressed flue gas at the        first elevated pressure from the first compressor to the first        gas/liquid contact device;    -   a conduit arrangement for recycling aqueous sulfuric acid        solution to the first gas/liquid contact column;    -   a second compressor for compressing SO₂-free carbon dioxide gas        to a second elevated pressure which is higher than the first        elevated pressure;    -   a conduit arrangement for feeding SO₂-free carbon dioxide gas        from the first counter current gas/liquid contact device to the        second compressor;    -   a second counter current gas/liquid contact device for washing        SO₂-free carbon dioxide gas with water at the second elevated        pressure for a sufficient time to produce SO₂-free, NO_(x)-lean        carbon dioxide gas and an aqueous nitric acid solution;    -   a conduit arrangement for feeding SO₂-free carbon dioxide gas at        the second elevated pressure from the second compressor to the        second gas/liquid contact device;    -   a conduit arrangement for recycling aqueous nitric acid solution        to the second gas/liquid contact device; and    -   a conduit arrangement for feeding SO₂-free, NO_(x)-lean carbon        dioxide gas from said second counter current gas/liquid contact        device to said gas drying system. The first and second        compressors are preferably stages of a multi-stage carbon        dioxide compression arrangement.

A “pressure reduction arrangement” is typically a pressure reductionvalve and the first, second, third and fourth pressure reductionarrangements are preferably separate pressure reduction valves.

In embodiments for the purification of waste gas from a hydrogen PSAsystem, the apparatus may comprise:

-   -   a hydrogen PSA system for separating crude hydrogen gas        comprising carbon dioxide and carbon monoxide to produce        hydrogen gas and waste carbon dioxide gas comprising carbon        monoxide;    -   a second compression arrangement for compressing waste carbon        dioxide gas to produce compressed waste carbon dioxide gas;    -   a conduit arrangement for feeding waste carbon dioxide gas from        the hydrogen PSA system to the second compression arrangement;    -   a gas dryer system for drying compressed waste carbon dioxide        gas to produce dried waste carbon dioxide gas;    -   a conduit arrangement for feeding compressed waste carbon        dioxide gas to the gas dryer system; and    -   a conduit arrangement for feeding dried waste carbon dioxide        gas, or a gas derived therefrom, the reboiler.

The heat exchanger is usually a multi-stream plate fin heat exchangerhaving a plurality of fluid passages in which cooling stream(s) flowcounter currently to warming stream(s). It is desirable that the feedstreams enter and the product streams leave the main heat exchangerusually at the most thermodynamically efficient locations. The heatexchanger is usually made from aluminum.

The present invention will now be described by way of example only andwith reference to FIGS. 2 and 3.

Much of the embodiment of the process of the present invention depictedin FIG. 2 is similar to the prior art process depicted in FIG. 1. Bothprocesses are for the recovery of carbon dioxide from flue gas generatedin an oxyfuel combustion process in power generation plant (not shown).The primary distinction between the prior art process of FIG. 1 and theprocess depicted in FIG. 2 is that phase separator C101 in FIG. 1 hasbeen eliminated and a distillation (or stripping) column C104 has beenadded.

Referring to FIG. 2, a stream 101 of waste gas, such as that of stream 1of the prior art process of FIG. 1 comprising about 73 mol % carbondioxide, is fed to a pair of thermally regenerated desiccant driers C103where it is dried to produce a stream 102 of contaminated carbon dioxidegas. Stream 102 is combined with a stream 117 of compressedoxygen-enriched gas recycled from downstream (see below) to form astream 103 of crude carbon dioxide gas. Stream 103 is cooled by indirectheat exchange in heat exchanger E101 against a stream 125 of carbondioxide-enriched liquid at a pressure of about 14.4 bar (see below) toproduce a stream 104 of crude gaseous carbon dioxide and a stream 126 ofcarbon dioxide-enriched gas.

Stream 104 is fed to reboiler E106 to reboil carbon-dioxide-enrichedbottoms liquid in column C104 to produce carbon dioxide-enriched vaporfor the column C104 and a stream 105 of cooled crude carbon dioxide gas,a portion of which may be condensed. Stream 105 is further cooled inheat exchanger E102 by indirect heat exchange to produce a stream 106 ofpartially condensed crude carbon dioxide gas. All of stream 106 is fedto a cold end phase separation vessel C102 operating at about −54° C.where it is separated into carbon dioxide-depleted vapor and impureliquid carbon dioxide.

A stream 107 of the carbon dioxide-depleted vapor is warmed to ambienttemperature in heat exchangers E102 and E101 by indirect heat exchangeto produce a stream 108 of carbon dioxide-depleted gas which is heatedby indirect heat exchange in pre-heater E103 to produce a stream 109 ofheated carbon dioxide-depleted gas at about 300° C. and about 30 bar.Stream 109 is work expanded in turbine K103 to produce power and astream 110 of expanded carbon dioxide depleted gas which is vented tothe atmosphere. Stream 110 comprises about 25 mol % carbon dioxide,about 53 mol % nitrogen, about 7 mol % argon, about 15 mol % oxygen andabout 13 ppm nitric oxide.

A stream 111 of the impure carbon dioxide liquid comprising about 95 mol% carbon dioxide, 1.1 mol % oxygen and about 3.7% total nitrogen andargon is removed from the phase separator C102, warmed to about −30° C.by indirect heat exchange in heat exchanger E102 to produce a stream 112of warmed impure carbon dioxide liquid and then expanded from about 30bar to about 16 bar in valve V103 to produce a stream 113 of expandedimpure carbon dioxide liquid which is fed to the top of the column C104.

The impure carbon dioxide liquid comprising about 1 mol % oxygen isseparated in column C104 to produce oxygen enriched-overhead vapor andcarbon dioxide-enriched bottoms liquid. The action of the strippingprocess is to reduce the oxygen concentration in the carbon dioxideextracted from the column to no more than 10 ppm and the nitrogen andargon level to about 280 ppm. The bottoms liquid is reboiled by indirectheat exchange against crude gaseous carbon dioxide in reboiler E106 (seeabove) to provide carbon dioxide-enriched vapor for the column.

The oxygen-enriched overhead vapor contains about 69% carbon dioxide,6.9% oxygen and 24.1% nitrogen plus argon. The carbon dioxideconcentration is too high to allow this vapor to be vented. Therefore, astream 114 of the oxygen-enriched overhead vapor is warmed by indirectheat exchange against cooling crude gaseous carbon dioxide in heatexchangers E102 and E101 to produce a stream 115 of warmedoxygen-enriched gas. Stream 115 is compressed from about 16 bar to about30 bar in compressor K104 to produce a stream 116 of compressedoxygen-enriched gas and the heat of compression removed by indirect heatexchange with a coolant, usually water, in aftercooler E107 to producethe stream 117 of compressed oxygen-enriched gas which is recycled tostream 102 (see above). The result of recycling stream 117 is that theentire portion of the separated gases is eventually discharged from theturbine K103 and vented to the atmosphere as stream 110.

A stream 118 of the carbon dioxide-enriched bottoms liquid is dividedinto two portions, stream 119 and stream 124. Refrigeration for theprocess is provided in part by expanding stream 119 to a pressure ofabout 5.6 bar in valve V102 to produce a stream 120 of expanded carbondioxide-enriched liquid and then vaporizing and warming stream 120 inheat exchangers E102 and E101 thereby producing a stream 121 of carbondioxide-enriched gas. Further refrigeration is provided by expandingstream 124 to a pressure of about 14.4 bar in valve 101 to produce astream 125 of expanded carbon dioxide-enriched liquid and thenvaporizing and warming stream 125 in heat exchanger E101 to produce astream 126 of carbon dioxide-enriched gas.

Streams 121 and 126 are compressed and combined in a multistagecentrifugal compressor K101, K102 to produce a stream 128 of compressedcarbon dioxide gas at a pressure of about 110 bar. The compressed carbondioxide gas comprises over 99.9 mol % carbon dioxide and only about 10ppm oxygen. The remaining portion consists of very small quantities ofnitrogen, argon and nitrogen oxides.

Carbon dioxide compressor K101, K102 is an integrally geared machinewith multiple radial stages. K101 has three or four stages, optionallywith intercooling between some stages although not within the last twostages because of the fact that the discharge pressure is above thecritical pressure. K102 is one or two stages of the same machine with anintercooler and an aftercooler.

In the exemplified embodiment, some or all of the stages of thecompressor K101, K102 are operated adiabatically and, thus, heat ofcompression is recoverable from the compressed carbon dioxide gas byindirect heat exchange with coolants using an intercooler E104 and anaftercooler E105. The coolant in intercooler E104 is water. The coolantin aftercooler E105 may be boiler feed water and/or condensate for thepower generation plant thus heat of compression can be used to pre-heatthese streams.

Stream 121 is compressed in the initial stage K102 of the compressor toproduce a stream 122 of compressed carbon dioxide gas. Heat ofcompression is removed from stream 122 by indirect heat exchanger withcooling water in intercooler E104 to produce a stream 123 of cooledcompressed carbon dioxide gas at a pressure of about 14.4 bar. Stream123 is combined with stream 126 and the combined stream is compressed inthe remaining stage(s) K101 of the compressor to produce a stream 127 offurther compressed carbon dioxide gas. Heat of compression is removedfrom stream 127 by indirect heat exchange with boiler feed water andthen condensate in aftercooler E105 to produce the stream 128 ofcompressed carbon dioxide gas at pipeline pressure, e.g. about 110 bar.K101 may also have at least one intercooler, cooled using cooling water,if it is not desirable to recover all of the heat to boiler feed waterand/or condensate.

The embodiment depicted in FIG. 3 is similar to the embodiment depictedin FIG. 2. The main difference between the two embodiments is that, inFIG. 3, three streams of expanded carbon dioxide-enriched liquid areused to provide refrigeration for the process rather than the twostreams used in the embodiment of FIG. 2. The same reference numeralshave been used in FIG. 3 as in FIG. 2 to denote the common featuresbetween the two embodiments. The following is a discussion of only theadditional features of the embodiment in FIG. 3.

Referring to FIG. 3, stream 118 of the carbon dioxide-enriched liquidfrom column C104 is divided into three portions; stream 119, stream 124and stream 129. Further refrigeration for the process is provided byexpanding stream 129 to a pressure of about 10 bar in valve 104 toproduce a stream 130 of expanded carbon dioxide-enriched liquid and thenvaporizing and warming stream 130 in heat exchanger E101 to produce astream 131 of carbon dioxide-enriched gas.

Streams 121, 126 and 131 are compressed and combined in a multistagecentrifugal compressor K101, K102A, K102B to produce a stream 133 ofcompressed carbon dioxide gas at a pressure of about 110 bar. Thecompressed carbon dioxide gas comprises 99.9 mol % carbon dioxide andonly about 10 ppm oxygen. The remaining portion consists of very smallquantities of nitrogen, argon and nitrogen oxides.

As in the embodiment depicted in FIG. 2, some or all of the stages K101,K102A, K102B of the compressor are operated adiabatically and, thus,heat of compression is recoverable from the compressed carbon dioxidegas by indirect heat exchange with coolants using intercoolers E104A,E104B and an aftercooler E105.

Heat of compression can be used in this way to pre-heat boiler feedwater and condensate. In this connection, stream 121 is compressed inthe initial stage(s) K102A of the compressor to produce a stream 122 ofcompressed carbon dioxide gas. Heat of compression is removed fromstream 122 by indirect heat exchanger with cooling water in intercoolerE104A to produce a stream 123 of cooled compressed carbon dioxide gas ata pressure of about 10 bar. Stream 123 is combined with stream 131 andthe combined stream is compressed in the intermediate stage(s) K102B ofthe compressor to produce a stream 127 of further compressed carbondioxide gas. Heat of compression is removed from stream 127 by indirectheat exchange with cooling water in intercooler E104B to produce stream128 of further compressed carbon dioxide gas at a pressure of about 17bar. Stream 128 is combined with stream 126 and compressed in the finalstage(s) K101 of the compressor to produce a stream 132 of compressedcarbon dioxide gas at a pressure of about 110 bar. Heat of compressionis removed from stream 132 by indirect heat exchange with boiler feedwater and then condensate in aftercooler E105 to produce stream 133 ofcompressed carbon dioxide.

Example 1

A computer simulation has been carried out using commercially availablesimulation software (Aspen Plus Version 2004.1) in which the processdepicted in FIG. 2 is integrated with an oxyfuel combustion process in apower generation plant. A heat and mass balance table for the simulationis provided in Table 2.

The simulation achieved the required level of carbon dioxide purity ofover 97 mol % (actually about 99.9 mol %), with about 87.4% carbondioxide recovery. However, the specific power consumption is increasedby 3% and carbon dioxide recovery reduced by 1.6% compared to the priorart process shown in FIG. 1.

A computer simulation (Aspen Plus Version 2004.1) of the same processbut vaporizing a third level of liquid carbon dioxide to provide furtherrefrigeration (FIG. 3) indicates that overall power consumption can bereduced by about 13% compared to the process depicted in FIG. 1.

TABLE 2 Stream Number 101 102 103 104 105 106 107 108 109 110Temperature ° C. 24.83 24.83 24.85 −4.08 −19.64 −53.70 −53.70 11.70300.00 62.76 Pressure bar a 30 30.00 30 30 30 30 30 30 30 1.1 Flow kg/s140.49 140.40 157.17 157.17 157.17 157.17 42.43 42.43 42.43 42.43Composition CO2 mol % 72.7633 72.8651 72.5987 72.5987 72.5987 72.598725.3191 25.3191 25.3191 25.3191 N2 mol % 18.9694 18.9959 18.8951 18.895118.8951 18.8951 52.4127 52.4127 52.4127 52.4127 Ar mol % 2.6956 2.69942.9277 2.9277 2.9277 2.9277 7.2751 7.2751 7.2751 7.2751 O2 mol % 5.43165.4392 5.5778 5.5778 5.5778 5.5778 14.9917 14.9917 14.9917 14.9917 H2Omol % 0.1396 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 SO2 ppm 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 NO ppm 4.9674 4.9744 5.6409 5.6409 5.6409 5.6409 13.140713.1407 13.1407 13.1407 NO2 ppm 0.0043 0.0043 0.0038 0.0038 0.00380.0038 0.0000 0.0000 0.0000 0.0000 Stream Number 111 112 113 114 115 116117 118 119 120 Temperature ° C. −53.70 −27.43 −36.99 −36.92 11.70 70.8325.00 −25.48 −25.48 −54.70 Pressure bar a 30 30 16.75936 16.7593616.75936 30 30 16.75936 16.75936 5.603787 Flow kg/s 114.74 114.74 114.7416.77 16.77 16.77 16.77 97.97 43.84 43.84 Composition CO2 mol % 95.222195.2221 95.2221 70.3742 70.3742 70.3742 70.3742 99.8876 99.8876 99.8876N2 mol % 2.8569 2.8569 2.8569 18.0534 18.0534 18.0534 18.0534 0.00360.0036 0.0036 Ar mol % 0.8475 0.8475 0.8475 4.8350 4.8350 4.8350 4.83500.0988 0.0988 0.0988 O2 mol % 1.0733 1.0733 1.0733 6.7362 6.7362 6.73626.7362 0.0100 0.0100 0.0100 H2O mol % 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 0.0000 0.0000 SO2 ppm 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 0.0000 0.0000 0.0000 NO ppm 2.0523 2.0523 2.052311.2086 11.2086 11.2086 11.2086 0.3331 0.3331 0.3331 NO2 ppm 0.00570.0057 0.0057 0.0001 0.0001 0.0001 0.0001 0.0067 0.0067 0.0067 StreamNumber 121 122 123 124 125 126 127 128 Temperature ° C. 11.70 92.9725.00 −25.48 −28.50 11.70 207.11 50.00 Pressure bar a 5.603787 15.1138315.11383 16.75936 15.11383 15.11383 110 110 Flow kg/s 43.84 43.84 43.8454.13 54.13 54.13 97.97 97.97 Composition CO2 mol % 99.8876 99.887699.8876 99.8876 99.8876 99.8876 99.8876 99.8876 N2 mol % 0.0036 0.00360.0036 0.0036 0.0036 0.0036 0.0036 0.0036 Ar mol % 0.0988 0.0988 0.09880.0988 0.0988 0.0988 0.0988 0.0988 O2 mol % 0.0100 0.0100 0.0100 0.01000.0100 0.0100 0.0100 0.0100 H2O mol % 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 SO2 ppm 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 NO ppm 0.3331 0.3331 0.3331 0.3331 0.3331 0.3331 0.33310.3331 NO2 ppm 0.0067 0.0067 0.0067 0.0067 0.0067 0.0067 0.0067 0.0067

Example 2

A computer simulation (Aspen Plus Version 2004.1) has been carried outin which the process depicted in FIG. 2 is integrated with a hydrogenPSA system (not shown). The off gas from the PSA system is compressed to30 bar to form a stream 101 of compressed off gas which is fed to theprocess. A heat and mass balance table for the simulation is provided inTable 3.

The simulation indicates that the carbon monoxide level can be reducedto about 100 ppm.

TABLE 3 Stream Number 101 102 103 104 105 106 107 108 109 110Temperature ° C. 20.00 20.00 20.30 −3.21 −16.35 −53.65 −53.65 8.40300.00 65.90 Pressure bar a 30 30.00 30 30 30 30 30 30 30 1.1 Flow kg/s54.59 54.56 59.05 59.05 59.05 59.05 9.78 9.78 9.78 9.78 Composition CO2mol % 71.6016 71.6799 72.4768 72.4768 72.4768 72.4768 23.7484 23.748423.7484 23.7484 N2 mol % 0.9951 0.9962 1.0183 1.0183 1.0183 1.01832.6859 2.6859 2.6859 2.6859 Ar mol % 0.1682 0.1684 0.1836 0.1836 0.18360.1836 0.4388 0.4388 0.4388 0.4388 H2 mol % 21.8609 21.8848 20.835520.8355 20.8355 20.8355 59.0303 59.0303 59.0303 59.0303 H2O mol % 0.10920.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 CO mol %4.5819 4.5869 4.7389 4.7389 4.7389 4.7389 12.3553 12.3553 12.355312.3553 CH4 mol % 0.6830 0.6838 0.7469 0.7469 0.7469 0.7469 1.74131.7413 1.7413 1.7413 Stream Number 111 112 113 114 115 116 117 118 119120 Temperature ° C. −53.65 −23.91 −31.45 −31.03 8.40 64.23 25.00 −25.11−25.11 −54.65 Pressure bar a 30 30 16.82649 16.82649 16.82649 30 3016.82649 16.82649 5.603904 Flow kg/s 49.27 49.27 49.27 4.49 4.49 4.494.49 44.78 18.76 18.76 Composition CO2 mol % 98.3195 98.3195 98.319583.8946 83.8946 83.8946 83.8946 99.9195 99.9195 99.9195 N2 mol % 0.13390.1339 0.1339 1.3351 1.3351 1.3351 1.3351 0.0006 0.0006 0.0006 Ar mol %0.0483 0.0483 0.0483 0.4007 0.4007 0.4007 0.4007 0.0092 0.0092 0.0092 H2mol % 0.5794 0.5794 0.5794 5.8025 5.8025 5.8025 5.8025 0.0000 0.00000.0000 H2O mol % 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 CO mol % 0.6995 0.6995 0.6995 6.9160 6.9160 6.9160 6.91600.0100 0.0100 0.0100 CH4 mol % 0.2195 0.2195 0.2195 1.6512 1.6512 1.65121.6512 0.0607 0.0607 0.0607 Stream Number 121 122 123 124 125 126 127128 Temperature ° C. 8.40 84.10 25.00 −25.11 −29.98 8.40 209.91 50.00Pressure bar a 5.603904 14.27814 14.27814 16.82649 14.27814 14.27814 110110 Flow kg/s 18.76 18.76 18.76 26.02 26.02 26.02 44.78 44.78Composition CO2 mol % 99.9195 99.9195 99.9195 99.9195 99.9195 99.919599.9195 99.9195 N2 mol % 0.0006 0.0006 0.0006 0.0006 0.0006 0.00060.0006 0.0006 Ar mol % 0.0092 0.0092 0.0092 0.0092 0.0092 0.0092 0.00920.0092 H2 mol % 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000H2O mol % 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 CO mol% 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 0.0100 CH4 mol %0.0607 0.0607 0.0607 0.0607 0.0607 0.0607 0.0607 0.0607

Advantages of preferred embodiments of the present invention include:

-   -   improving low temperature carbon dioxide purification;    -   producing carbon dioxide at a purity of at least 97 mol %, and        usually at least 99 mol %, e.g. 99.9 mol %;    -   producing carbon dioxide with a very low level of oxygen or        carbon monoxide, e.g. no more than 1000 ppm, typically no more        than 100 ppm, and usually about 10 ppm (or even lower if        required);    -   producing carbon dioxide with very low levels of nitrogen and        argon or other contaminants, typically a combined level of no        more than 1000 ppm;    -   minimal or no increase in overall power consumption compared        with the prior art process of FIG. 1 (defined as kWh/tonne of        carbon dioxide separated); and    -   minimal or no decrease in recovery of carbon dioxide compared        with the prior art process of FIG. 1.

It will be appreciated that the invention is not restricted to thedetails described above with reference to the preferred embodiments butthat numerous modifications and variations can be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. Apparatus for removing a first contaminant selected from oxygen andcarbon monoxide from impure liquid carbon dioxide, said apparatuscomprising: a mass transfer separation column system for separatingimpure liquid carbon dioxide to produce first contaminant-enrichedoverhead vapor and carbon dioxide-enriched bottoms liquid; a reboilerfor re-boiling carbon dioxide-enriched bottoms liquid by indirect heatexchange against crude carbon dioxide fluid to produce carbondioxide-enriched vapor for said column system and cooled crude carbondioxide fluid; a heat exchanger for further cooling cooled crude carbondioxide fluid by indirect heat exchange to produce partially condensedcrude carbon dioxide fluid; a conduit arrangement for feeding cooledcrude carbon dioxide fluid from said reboiler to said heat exchanger; aphase separator for phase separating said partially condensed crudecarbon dioxide fluid to produce said impure liquid carbon dioxide andcarbon dioxide-depleted vapor; a conduit arrangement for feedingpartially condensed crude carbon dioxide fluid from said heat exchangerto said phase separator; a first pressure reduction arrangement forreducing the pressure of impure liquid carbon dioxide to produce reducedpressure impure liquid carbon dioxide; a conduit arrangement for feedingimpure liquid carbon dioxide from said phase separator to said firstpressure reduction arrangement; and a conduit arrangement for feedingreduced pressure impure liquid carbon dioxide from said first pressurereduction arrangement to said column system.
 2. Apparatus according toclaim 1 wherein said conduit arrangement for feeding impure liquidcarbon dioxide from said phase separator to said first pressurereduction arrangement comprises: a conduit arrangement for feedingimpure liquid carbon dioxide from said phase separator to said heatexchanger for warming to provide warmed impure liquid carbon dioxide; atleast one fluid passage in said heat exchanger; and a conduitarrangement for feeding warmed impure liquid carbon dioxide from saidheat exchanger to said first pressure reduction arrangement. 3.Apparatus according to claim 1 comprising: a second pressure reductionarrangement for expanding carbon dioxide-enriched bottoms liquid toproduce expanded carbon dioxide-enriched bottoms liquid at a firstpressure; a conduit arrangement for feeding carbon dioxide-enrichedbottoms liquid from said column system to said second pressure reductionarrangement; and a conduit arrangement for feeding expanded carbondioxide-enriched bottoms liquid at said first pressure from said secondpressure reduction arrangement to said heat exchanger for vaporizationto provide refrigeration duty.
 4. Apparatus according to claim 3comprising: a third pressure reduction arrangement for expanding carbondioxide-enriched bottoms liquid to produce expanded carbondioxide-enriched bottoms liquid at a second pressure which is higherthan said first pressure; a conduit arrangement for feeding carbondioxide-enriched bottoms liquid from said column system to said thirdpressure reduction arrangement; and a conduit arrangement for feedingexpanded carbon dioxide-enriched bottoms liquid at said second pressurefrom said third pressure reduction arrangement to said heat exchangerfor vaporization to provide refrigeration duty.
 5. Apparatus accordingto claim 4 comprising: a fourth pressure reduction arrangement forexpanding carbon dioxide-enriched bottoms liquid to produce expandedcarbon dioxide-enriched bottoms liquid at a third pressure which ishigher than said first pressure and lower than said second pressure; aconduit arrangement for feeding carbon dioxide-enriched bottoms liquidfrom said column system to said fourth pressure reduction arrangement;and a conduit arrangement for feeding said expanded carbondioxide-enriched bottoms liquid at said third pressure from said fourthpressure reduction arrangement to said heat exchanger for vaporizationto provide refrigeration duty.
 6. Apparatus according to claim 1comprising: a conduit arrangement for feeding first contaminant-enrichedoverhead vapor from said column system to said heat exchanger forwarming to provide warmed first contaminant-enriched gas; a recyclecompressor arrangement for compressing warmed first contaminant-enrichedgas to produce compressed first contaminant-enriched gas; a conduitarrangement for feeding warmed first contaminant-enriched gas from saidheat exchanger to said recycle compressor arrangement; a conduitarrangement for combining compressed first contaminant-enriched gas fromthe compressor arrangement with contaminated carbon dioxide gas to formcrude carbon dioxide gas; a conduit arrangement for feeding said crudecarbon dioxide gas from said conduit arrangement combining saidcontaminated gases to said heat exchanger for cooling to provide crudecarbon dioxide fluid; and a conduit arrangement for feeding crude carbondioxide fluid from said heat exchanger to said reboiler.
 7. Apparatusaccording to claim 6 wherein said conduit arrangement for combining saidcontaminated gases comprises: an aftercooler for removing heat ofcompression from compressed first contaminant-enriched gas by indirectheat exchange with a coolant to produce cooled compressed firstcontaminant-enriched gas; a conduit arrangement for feeding compressedfirst contaminant-enriched gas from said recycle compressor arrangementto said aftercooler; a conduit arrangement for combining cooledcompressed first contaminant-enriched gas from said aftercooler withsaid contaminated carbon dioxide gas.